1. Field of the Invention
This invention relates generally to oscillators and, more specifically, to an IC (current-capacitor) precision oscillator which allows the user to have control over the frequency and the duty cycle of the precision oscillator by altering an input current of the precision oscillator.
2. Description of the Prior Art
Oscillator circuits are used in a myriad of applications in the electronics industry for providing clock and other timing signals to electronic circuitry such as microprocessors, microcontrollers, flip-flop circuits, latch circuits, etc. Typical oscillator circuits include a control circuit coupled to an interconnection between a series resistor-capacitor (RC) network (i.e., RC oscillator). The control circuit alternately charges or discharges the voltage across the capacitor through the resistor to generate an oscillatory signal appearing across the capacitor. The frequency of oscillation is determined by the time constant of the resistor and the capacitor.
One technique for building an RC oscillator is to use a conventional NE555 timer (hereinafter the 555 timer), manufactured by National Semiconductor, as the circuit that controls the charging and discharging of the capacitor of the RC network. The 555 timer includes a set/reset (SR) flip-flop and first and second comparators. The interconnection between the series RC network is coupled to one input of each of the comparators. The other input of the first comparator is coupled to receive a high threshold voltage (V.sub.H) while the other input of the second comparator is coupled to receive a low threshold voltage (V.sub.L). The output of the first comparator is coupled to the set input of the flip-flop while the output of the second comparator is coupled to the reset input of the flip-flop. An output of the flip-flop is coupled to the resistor of the RC network.
In operation, the first comparator sets the flip-flop, which commences the discharging of the voltage across the capacitor, when the RC oscillatory signal exceeds the predetermined high threshold voltage, and the second comparator resets the flip-flop, which commences the charging of the voltage across the capacitor, when the RC oscillatory signal falls below the predetermined low threshold voltage. In this manner, the signal appearing across the capacitor approximately oscillates between the high and the low threshold voltages at a frequency determined by the value of the resistor capacitor of the RC network.
The problem with the above embodiment is that the configuration suffers from the drawback that by the time the flip-flop is set (or reset) in response to the switching of one of the comparators, the RC oscillatory signal has actually risen above the high threshold voltage (in the case of setting the flip-flop) or has fallen below the low threshold voltage (in the case of resetting the flip-flop). As a result, variations in the frequency of oscillation occur because the RC oscillatory signal does not accurately oscillate between the desired high and low threshold voltages. Such error is unacceptable when an accurate oscillatory signal is required.
Present RC oscillators also do not allow the user to vary the duty cycle. Furthermore, current RC oscillators do not have very broad operating frequency ranges. In order to broaden the operating frequency range of present RC oscillators, significant changes to the RC network would have to be made.
Therefore, a need existed to provide an improved oscillator. The improved oscillator must be a precision oscillator. The precision oscillator must generate an output frequency which varies within +/- 3% of the desired frequency level. The precision oscillator must have a wide programmable frequency range without requiring significant changes to the circuitry of the precision oscillator. The precision oscillator must also allow for an adjustable duty cycle.